Cerebral Effects of Nitrous Oxide When Added to Low and High Concentrations of Isoflurane in the Dog

The purpose of this canine study was to examine the cerebral vascular and metabolic effects of adding nitrous oxide to isoflurane from 1.4% expired (1 MAC) up to a concentration giving an isoelectric electroencephalogram (EEG). Cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRo2) were determined using a sagittal sinus outflow technique. At 2.4% expired isoflurane, 70% nitrous oxide increased CBF but had no effect on CMRo2. At 3.1% expired isoflurane, 70% nitrous oxide had no effect on either CBF or CMRo2. The latter concentration of isoflurane rendered the EEG isoelectric, but when nitrous oxide was added, EEG activity reappeared. To again produce an isoelectric EEG required an increase in the isoflurane concentration to 3.5% ± 0.2% (with no further effect on CMRo2). The authors also found that at 1.4% isoflurane, 0.9 μg·kg·min of angiotensin significantly decreased CMRo2 without any effect on CBF. It is concluded that nitrous oxide, when added to isoflurane concentrations ranging from 1.4% to 3.5% in the dog, increases CBF at the low but not the high isoflurane concentrations although it has no effect on CMRo2.